RF system for an MRI apparatus, provided with bead-shaped spacers

ABSTRACT

RF connection cable  2  for interconnecting an RF coil and an RF processing apparatus in an RF system of a medical MRI apparatus. The connection cable  2  is comprised of an outer sleeve  6  enclosing an inner cable comprising RF conductors  24 , an inner sleeve  20  and a shield  22 . Between the inner sleeve  20  and the outer sleeve  6  a plurality of spacers in the form of large beads  12 - i  alternating with small beads  14 - i  are provided. In this way, a very flexible cable is obtained having comparatively low dielectric losses and a low capacitive coupling with the patient.

BACKGROUND

The invention relates to an RF system for use in a medical MRIapparatus, provided with an RF coil and an RF connection cable connectedto said RF coil, which connection cable comprises a number of RFconductors and a number of spacers in the form of drilled-through beadswhich are provided around said RF conductors and into which the RFconductors extend through the drilled holes in the spacers.

In a medical MRI apparatus, movable RF coils are used for receiving RFsignals generated in the tissue of a patient to be examined. Said RFcoils are connected, via an RF connection cable, to the RF processingequipment of the MRI apparatus in order to transfer the RF signalsinduced in the RF coil to said equipment. It is a well-known phenomenonthat under certain conditions the signals passing through these RFconductors may be harmful to the patient, in particular cause burns, ifthe RF connection cable extends close to the skin of the patient.

Swiss patent specification No. 192668 discloses a high-frequency cablewherein an RF conductor is surrounded by a number of bead-shapedspacers, which are spaced apart and rigidly provided on the RFconductor. RF cables for medical MRI applications often require aplurality of conductors, for example four, as a result of which rigidlyattaching the spacers to all RF conductors is complicated.

SUMMARY

It is an object of the invention to provide an RF connection cable foruse in a medical MRI apparatus, which connection cable can bemanufactured in a simple manner. For this purpose, the RF system inaccordance with the invention is characterized in that the RF connectioncable is provided with an outer sleeve, and the bead-shaped spacers havealternately a first dimension and a second dimension.

By providing an outer sleeve, the RF connection cable is made suitablefor medical applications and, in particular, any direct contact betweena part of the patient's body and the RF conductors is precluded. Byproviding alternately larger and smaller bead-shaped spacers, the cableobtained is highly flexible as a result of which the ease of handlingfor the operating staff is improved. A first additional advantage isthat RF conductors of a first RF cable cannot get close to RF conductorsof another RF cable situated in the vicinity, so that the mutualelectrical influence, if any, is small. A second additional advantage isthat a part of the volume between the RF conductors and the outer sleeveis formed by air, as a result of which the dielectric losses in theintermediate space are smaller than they would be in a situation wherethis space is entirely filled with the material of the spacers. Inaddition, the capacitive coupling between the patient's body and the RFconductors is reduced thereby, so that the risk of so-termed “hot spots”on the patient's body at locations where it contacts the connectioncable is reduced, and image artefacts in these locations arecounteracted.

In a preferred embodiment of the invention, the bead-shaped spacers aremade of polyoxymethylene or of polycarbonate. By virtue thereof, theeffect of the above-described low electrical losses, low capacitivecoupling and reduction of image artefacts is optimized.

In another embodiment of the invention, the outer sleeve is providedwith a smooth outside surface. In addition, the outer sleeve may be madeof a biocompatible and biostable synthetic resin. By virtue thereof, thecable can be readily cleaned and is excellently suited for medicalapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

The invention will be described hereinafter with reference to theFigures, wherein corresponding parts are indicated by means of the samereference numerals. In the drawings:

FIG. 1 diagrammatically shows an RF connection cable in accordance withthe invention;

FIG. 2 is a cross-sectional view of the RF connection cable inaccordance with the invention.

DETAILED DESCRIPTION

FIG. 1 diagrammatically shows an RF connection cable in accordance withthe invention. The RF connection cable 2 is used in an RF system, whichRF system is comprised of at least one RF coil, at least one RFconnection cable per coil and RF processing equipment. The RF connectioncable 2 is used to connect the RF coil (not shown in the Figure) for usein a medical MRI apparatus to the RF processing equipment of said MRIapparatus. To this end, the RF coil is connected to one end 4 and the RFprocessing equipment (not shown) is connected to the other end 6 withthe help of customary connectors.

The outside of the connection cable 2 is formed by an outer sleeve 8with a smooth outside surface enabling said cable to be readily cleaned.The material from which the outer sleeve 8 is made is preferablybiocompatible and biostable, i.e. it must not react with tissue of thepatient to be examined or be influenced itself by said tissue. Amaterial which can suitably be used for this purpose is PVC, which canbe obtained, in a form suitable for this purpose, from the firm of AdolfDamerius, Schrobenhausen, Germany.

A number of RF conductors, diagrammatically indicated by means of aninterrupted line 10, is accommodated in the interior of the RFconnection cable 2. Between the RF conductors 10 and the outer sleeve 8,a number of bead-shaped spacers 12-1, 12-2 . . . 12-i . . . are providedhaving a first external dimension, which is comparatively large in thiscase, for example 12 mm. Between these spacers 12-i there is provided anumber of bead-shaped spacers 14-1, 14-2 . . . 14-i . . . having adifferent external dimension, which is comparatively small in this case,for example 8 mm. The spacers 14-i and the spacers 12-i are alternatelyarranged. In this manner, a high degree of flexibility of the RFconnection cable 2 is achieved; in addition, a number of cavities 16-1,16-2 . . . which are not filled with dielectric material are formed inthis manner, as a result of which the dielectric losses and undesirablecapacitive couplings are reduced. The bead-shaped spacers 12-i and 14-iare all provided with a hole 18-1, 18-2 through which the RF conductors10 are fed.

The material from which the bead-shaped spacers 12-i and 14-i are madeis preferably polyoxymethylene (POM) or polycarbonate, which materialsare light and tenacious, i.e. they have a favorable influence on theease of handling and the strength of the cable, while they do not causethe image produced by the MRI apparatus to be disturbed.

FIG. 2 is a cross-sectional view of the RE connection cable 2 inaccordance with the invention. Inside the outer sleeve 8 comparativelylarge bead-shaped spacers 12-i are provided which alternate withcomparatively small bead-shaped spacers 14-1, which spacers are allprovided with a hole 18-i. Through the holes an inner cable is providedwhich is comprised of (from the outside inwards, successively) asynthetic resin (PVC) inner sleeve 20, an electrical RF shield 22 of,for example, copper and four RF conductors 24; the space between the RFconductors 24 within the shield 22 is filled with a suitable material 26for, inter alia, strain relief, for example a combination of fabric andsynthetic resin. The assembly of inner sleeve and all parts accommodatedtherein is commercially available as an assembled cable from the firm ofErnst & Engbring, Oer-Erkenschwick, Germany.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be constructed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. An RF system for use in a medical MRI apparatus, provided with an RF coil and an RF connection cable connected to said RF coil, which RF connection cable comprises: a plurality of RF conductors; an RF shield surrounding the plurality of conductors; a plurality of spacers in the form of drilled-through beads provided around said RF shield, the RF conductors and the RF shield extending through drilled holes in the bead-shaped spacers, the bead-shaped spacers having alternately a first dimension and a second dimension; an outer sleeve provided around the spacers.
 2. An RF system as claimed in claim 1, wherein the bead-shaped spacers are made of polyoxymethylene or of polycarbonate.
 3. An RF system as claimed in claim 1, wherein the outer sleeve is provided with a smooth outside surface.
 4. An RF system as claimed in claim 3, wherein the outer sleeve is made of a biocompatible and biostable synthetic resin.
 5. An RF connection cable for use in an RF system for a medical MRI apparatus, as defined in claim
 1. 6. An RF system as claimed in claim 1, wherein the bead-shaped spacers are made of a material with low dielectric losses, with low capacitive coupling, and which reduces MR image artifacts.
 7. An RF system as claimed in claim 1 further including: a dielectric material disposed inside the RF shield and around the RF conductors.
 8. An RF system as claimed in claim 7, further including: a resin sleeve between the RF shield and the bead-shaped spacers.
 9. A magnetic resonance imaging system including the RF connection cable as claimed in claim 1 connecting a movable RF coil with other components of the magnetic resonance imaging system and providing an RF communication link therebetween.
 10. An RF cable which extends closely adjacent a patient to connect an MRI apparatus with a movable RF coil, the RF coil tending to couple capacitively with the patient and potentially causing burns, the RF cable comprising: a plurality of RF conductors encased in a dielectric material; an RF shield surrounding the RF conductors; a plurality of beads of a material that reduces capacitive coupling, the beads having holes through which the RF shield and conductors are threaded; a smooth biocompatible sheath surrounding the beads, the beads being of a plurality of sizes such that air gaps are defined between the beads and the sheath to reduce the capacitive coupling with the patient.
 11. An RF cable as claimed in claim 10, further including: an insulating sleeve surrounding the RF shield.
 12. An RF cable as claimed in claim 11, wherein the bead-like holes have a larger diameter than the insulating sleeve.
 13. In combination a movable RF coil and an RF cable as claimed in claim
 10. 14. A magnetic resonance imaging system including: a movable RF coil; an RF cable as claimed in claim 10 connecting the RF coil with the magnetic resonance imaging system. 